Double-wall door panels composed of an outer plate and an inner plate are commonly found in, e.g., side doors for industrial equipment. The door panel's inner plate has a plurality of bottom portions and ridge portions. The bottom portions of the inner plate are bonded to the outer plate with adhesive. Then, an outer edge portion of the outer plate is bent inward to roll over an outer edge portion of the inner plate, and then pressed (hemming operation). The above is a method of fixing the outer plate and the inner plate to each other (see Patent Literature 1 for example).
In the double-wall door panel structure of this type, a foaming material may be placed between the outer plate and the ridge portions of the inner plate, to fill the inner space for reducing vibration and noises.